The present invention relates to turbomachinery and is concerned more particularly with apparatus for controlling clearance between a rotor and stator elements in such machinery.
An inherent problem in gas turbine power plants is the maintenance of a close or tight running clearance between the rotating and non-rotating structures in compressor sections as well as turbine sections. The problem also exists in ducted turbofan engines between the fan elements and the shroud or duct surrounding the fan. A tight clearance is desired during all modes of engine operation in order to achieve and hold high performance with efficiency during an extended lifetime of the engine.
The problem, however, of maintaining a tight clearance originates from a number of sources. Manufacturing and assembly tolerances dictate that at least some running clearance be provided between the radially inner ends of the stator vanes and the radially outer ends of the rotor blades in a conventional axial flow engine. Other sources related to engine operation are equally or more significant than the manufacturing tolerances. Transient thermal growths of the stator and rotor components are not equal during all modes of engine operation and, therefore, adequate clearance must be provided to prevent destructive interference until a steady state condition is reached. Furthermore, in the steady state condition thermal expansion of both the rotor and stator components and inertial growth during changing power demands must be accommodated by the clearance. Still further, flight maneuver deflections of the rotor and casing add further to the difficulty of establishing a tight clearance between the rotating and non-rotating structures.
The clearance difficulties cannot be solved by simply maintaining a large space between the stator and rotor in the gas flow path because gas leakage past the vanes results in a loss of static pressure or gas velocity and reduces the power output of the engine with a corresponding increase in specific fuel consumption.
Accordingly, it is a general object of the present invention to provide an improved clearance control apparatus for maintaining a desired or preselected clearance between the rotating and non-rotating components in a turbomachine. It is a further object of the invention to provide apparatus within turbomachinery which, in addition to maintaining clearance control, also offers the opportunity of reducing rotor vibration.